HTML: Server-Sent Events (SSE) API with Examples

How to use SSE API - Server-Sent Events

HTML: Server-Sent Events (SSE)

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Server-Sent Events (SSE) permit a web page to obtain updates from a server.

Server-Sent Events - One Way Messaging

A server-sent event is when a web page automatically obtains updates from a server.

This was even possible before, but the web page would have to ask if any updates were available. With server-sent events, the updates arrive automatically.

Examples: Facebook/Twitter updates, stock price updates, news feeds, sports results, etc.

Receive Server-Sent Event Notifications

The EventSource object is utilised to obtain server-sent event notifications:


<!DOCTYPE html>

<h3>Getting server updates</h3>

<div id="result"></div>

if(typeof(EventSource) !== "undefined") {
  var source = new EventSource("demo_sse.php");
  source.onmessage = function(event) {
    document.getElementById("result").innerHTML += + "<br>";
} else {
  document.getElementById("result").innerHTML = "Sorry, your browser does not support server-sent events...";


Example explained:

  • Make a new EventSource object, and identify the URL of the page sending the updates (in this example, "demo_sse.php")
  • Each time an update is obtained, the onmessage event occurs
  • When an onmessage event occurs, put the obtained data into the element with id="result"

Check Server-Sent Events Support

In the tryit example above, there were some additional lines of code to review browser support for server-sent events:

if(typeof(EventSource) !== "undefined") {
  // Yes! Server-sent events support!
  // Some code.....
} else {
  // Sorry! No server-sent events support..

Server-Side Code Example

For the example above to work, you require a server capable of sending data updates (like PHP or ASP).

The server-side event stream syntax is straightforward. Set the "Content-Type" header to "text/event-stream". Now you can start sending event streams.

Code in PHP (demo_sse.php):

header('Content-Type: text/event-stream');
header('Cache-Control: no-cache');

$time = date('r');
echo "data: The server time is: {$time}\n\n";

Code in ASP (VB) (demo_sse.asp):

Response.ContentType = "text/event-stream"
Response.Expires = -1
Response.Write("data: The server time is: " & now())

Code explained:

  • Specify the "Content-Type" header to "text/event-stream"
  • Determine that the page should not cache
  • Output the data to send ( always start with "data: " )
  • Flush the output data back to the web page

The EventSource Object

In the examples above, we utilised the onmessage event to obtain messages. But other events are also available:

onopenWhen a connection to the server is opened
onmessageWhen a message is received
onerrorWhen an error occurs


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